linux-kernel - Re: [PATCH] pvclock: introduce seqcount-like API

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Message-ID: <20160609133529.GA5400@rkaganb.sw.ru>
Date:	Thu, 9 Jun 2016 16:35:29 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<mnghuan@...il.com>, <luto@...nel.org>
Subject: Re: [PATCH] pvclock: introduce seqcount-like API

On Thu, Jun 09, 2016 at 02:47:54PM +0200, Paolo Bonzini wrote:
> On 09/06/2016 14:43, Roman Kagan wrote:
> > On Thu, Jun 09, 2016 at 01:23:23PM +0200, Paolo Bonzini wrote:
> >> The version field in struct pvclock_vcpu_time_info basically implements
> >> a seqcount.  Wrap it with the usual read_begin and read_retry functions,
> >> and use these APIs instead of peppering the code with smp_rmb()s.
> >> While at it, change it to the more pedantically correct virt_rmb().
> >>
> >> With this change, __pvclock_read_cycles can be simplified noticeably.
> >>
> >> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> >> ---
> >>  arch/x86/entry/vdso/vclock_gettime.c | 25 +++++------------------
> >>  arch/x86/include/asm/pvclock.h       | 39 +++++++++++++++++++++---------------
> >>  arch/x86/kernel/pvclock.c            | 17 ++++++----------
> >>  3 files changed, 34 insertions(+), 47 deletions(-)
> > [...]
> >> --- a/arch/x86/include/asm/pvclock.h
> >> +++ b/arch/x86/include/asm/pvclock.h
> > [...]
> >> @@ -69,23 +87,12 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
> >>  }
> >>  
> >>  static __always_inline
> >> -unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
> >> -			       cycle_t *cycles, u8 *flags)
> >> +cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src)
> >>  {
> >> -	unsigned version;
> >> -	cycle_t offset;
> >> -	u64 delta;
> >> -
> >> -	version = src->version;
> >> -	/* Make the latest version visible */
> >> -	smp_rmb();
> > 
> > This is on top of Minfei's patch, right?  It isn't in Linus' tree yet so
> > I wonder if it makes sense to merge the two patches into one.
> > 
> > Will you post it to stable, too?
> 
> Not this one, because Minfei's patch is enough to fix the bug, but I do

Has it landed in any public tree?  I'm unable to find any.  There
appears to be another version of the patch on the list, so I'm confused.

Roman.